This application proposes continuation of the very successful annual A.S.P.E.N. Research Workshop, funded in part in prior years by the NIDDK. The proposal is a 5-year application for a U13 award. The workshop has and will continue to be organized in close coordination with NIDDK staff (Drs. Van Hubbard and Michael Kenneth May). The workshop format is to bring together for one day >_100scientists interested and active in specific areas of clinical nutrition. The workshop is scheduled just prior to the annual Nutrition Week, at which A.S.P.E.N. holds its Clinical Congress. The workshop faculty include a Chair, Co-chair, and up to 8 speakers. The Chairs and speakers are all active scientists in disciplines of both basic and clinical research. Through active discussion between and among speakers and participants, we hope to identify each year promising areas for future investigation and collaboration on the chosen topic in clinical nutrition. Active participation of young scientists and research fellows will be encouraged by providing special travel funds for their attendance at the workshop. We have also established an abstract award mechanism targeted specifically toward younger investigators submitting abstracts related to the workshop topic each year. These mechanisms are designed to foster excitement and active participation of younger scientists in the workshop and will allow junior investigators to present work that will be seen by the distinguished speakers. The topic for the proposed 1styear (2003) workshop is Using Tracers to Measure Carbohydrate, Protein, and Fat Metabolism in Humans. Both radioactively and stable isotopically labeled compounds are used as "tracers" for specific substrates. In the last decade we have made significant advances, especially in the area of synthesis of stable isotopes at economical prices and in mass spectrometry for measuring these tracers. These innovations have led to the development of a range of sophisticated tracer methods that can be used to measure various aspects of substrate metabolism in humans. The purpose of this workshop is to review and discuss these methods and their applications to the measurement of the key substrates for energy metabolism in humans. Particular attention will be paid to more recent methods that allow us to measure not only the flux of individual metabolites in the body but also the rates of conversion of substrates from one group to another, e.g. glucose into fat or amino acids into glucose. These methods allow us to address interactions of substrates in ways unheard of only a few years ago and to design and test complex clinical nutrition formulations in a rational and fundamental way.